The bacterial genus Ca. Phytoplasma consists of relatively small bacteria, all of which are phloem-limited, non-helical and wall-less plant pathogens. Unlike viruses, phytoplasmas have their own, highly reduced metabolism although some of the molecules essential for their survival are acquired from the host. They represent a monophyletic Glade within the class Mollicutes, which is currently divided into at least 15 subgroups on the basis of sequence analyses of various conserved genes.
Grapevine yellows (GY), a complex of diseases that were originally thought to be caused by viruses, are now known to have a phytoplasma etiology. Almost identical symptoms of the GY syndrome are caused by different phytoplasmas and appear on leaves, shoots and clusters of grapevines (Vitis vinifera). Typical symptoms include discoloration and necrosis of leaf veins and leaf blades, downward curling of leaves, lack or incomplete lignifications of shoots, stunting and necrosis of shoots, abortion of inflorescences and shriveling of berries. Those symptoms are related to callose deposition at the sieve plates and subsequent degeneration of the phloem. Although no resistant cultivars of V. vinifera or rootstocks are known so far, the various grape varieties differ considerably as far as symptom severity is concerned. It ranges from fast decline and death in highly susceptible cultivars to tolerant rootstocks as symptomless carriers of the pathogen.
Approximately 70% of world grape production is used for wine, which is the highest value-added agri-food product in the world. The primary factor influencing wine quality is the condition under which the grapes are grown. Diseases, pests and viticultural practices greatly impact the consistency of the grape production and quality. Most of the time, diseases and pests are controlled with pesticides, either preventively or responsively.
The control of pathogens including but not limited to phloem-restricted pathogens (e.g., phytoplasma) needs new strategies given that conventional application were proven inefficient.